Titanium alloys are frequently used in aerospace and aeronautical applications because of the superior strength, low density, and corrosion resistance of titanium alloys. Titanium and its alloys exhibit a two-phase behavior. Pure titanium exists in an alpha phase having a hexagonal close-packed crystal structure up to its beta transus temperature (about 1625.degree. F.). Above the beta transus temperature, the structure changes to the beta phase having a body-centered-cubic crystal structure. Pure titanium is quite weak and highly ductile, but can achieve high strength and workable ductility when alloyed with other elements. Certain alloying elements also affect the behavior of the crystal structure, causing the alloy to behave either as an alpha or near-alpha alloy or as an alpha-beta alloy at room temperature. Alpha-beta alloys are made by adding one or more beta stabilizers, such as vanadium, which inhibit the transformation from beta to alpha and depress the beta transus temperature such that the alloy exists in a two-phase alpha-beta form at room temperature. Alpha alloys are made by adding one or more alpha stabilizers, such as aluminum, which raise the beta transus temperature and stabilize the alpha form such that the alloy is predominately in the alpha form at room temperature.
Two basic types of titanium alloys are currently in use in the rocket propulsion industry: Ti-6-4, an alpha-beta alloy consisting principally of about 6 percent aluminum, 4 percent vanadium, and the balance titanium and incidental impurities; and Ti-5-2.5, a near-alpha alloy consisting principally of about 5 percent aluminum, 2.5 percent tin, and the balance titanium and incidental impurities. The Ti-6-4 alloy is more readily available and is more easily processed to final form than the Ti-5-2.5 alloy, making Ti-6-4 much less costly than Ti-5-2.5.
Very low-temperature applications, such as for hydrogen fuel pumps or the like, impose severe restrictions on the types of alloys that can be used, primarily because the notch sensitivity of an alloy can be degraded to unacceptable levels at such temperatures. Of the currently available commercially produced alloys, Ti-5-2.5 ELI (Extra Low Interstitial grade processed to have reduced incidence of interstitial impurities) is currently the alloy of choice for cryogenic temperature applications because of its relatively high ultimate strength (on the order of 210 ksi) and its relatively high notch tensile ratio or NTR (on the order of 1.1) at liquid hydrogen temperatures of about 20K. The NTR is defined as the ultimate tensile strength of a notched test specimen divided by the ultimate tensile strength of a smooth test specimen, and is a standard measure of the notch sensitivity of a material. The more common, stronger, and less costly Ti-6-4 ELI alloy is known to have poor ductility and be notch sensitive (i.e., its NTR is less than 1.0) at cryogenic temperatures of 77K and below, and thus is a less favorable choice.
It would be desirable, however, to be able to use the stronger Ti-6-4 alloy in cryogenic and other applications, rather than the Ti-5-2.5 alloy, because Ti-6-4 is significantly less costly. Additionally, there is typically a very long lead time for purchase of Ti-5-2.5 ELI because there currently are only two known significant domestic users of this alloy. Accordingly, use of Ti-6-4 would enable quicker turnaround times. Furthermore, it would be desirable to provide a titanium alloy having improved ultimate strength compared to both Ti-5-2.5 and standard Ti-6-4, and having an acceptable NTR, preferably at least 1.0, at cryogenic temperatures. To achieve these ends, however, a non-standard processing of the standard Ti-6-4 alloy would be required in order to improve the strength and NTR at cryogenic temperatures.
It is known from the Hall-Petch relationship in physical metallurgy that decreasing the grain size results in an increase in strength. There is no known generally applicable correlation between grain size and NTR in alpha-beta titanium alloys, and very little data are available on how the properties of alpha-beta alloys behave as a function of grain size at cryogenic temperatures. There are some data to suggest, however, that at least in steels, an equiaxed grain size reduction can lead to both an increase in strength and an increase in fracture toughness.
Standard mill practice for Ti-6-4 bar calls for forging to occur at a temperature where the alloy is in the 2-phase alpha-beta field. The primary alpha that exists at these temperatures, typically in the range of 1600 to 1750.degree. F., pins the beta grains during the deformation and leads to an initial grain size refinement. Following forging, the alloy is cooled to room temperature, which results in the decomposition of the high temperature beta grains to a lenticular mixture of alpha and beta through nucleation and growth processes. Thus, the final microstructure consists of relatively large "primary" alpha grains, on the order of 10 to 50 .mu.m, and a fine mixture of alpha and beta plates whose scale is dependent on cooling rate (i.e., finer as the cooling rate increases).
One method for attaining finer grain sizes would be to use dynamic recrystallization during hot working. This is the process that leads to the initial refinement of the beta grains during conventional forging of alpha-beta alloys described above. However, the alpha grains do not change size during conventional forging and they do not undergo recrystallization with increased strain. Accordingly, it is impossible to attain a uniform fine grain size with the conventional forging process for Ti-6-4 because of the presence of the primary alpha grains.